In reply to a post by Pheasant:

As said, the ONT is basically a physical layer and protocol bridge. Well its a bit more than that - but for the purposes here that is its basic function.

IPv4, IPv6, PPP, NAT, CGNAT etc etc are constructs and concepts outside of this. They are in the layers that sit above.

Thanks, this is clear now.

Regarding the IP, my guess is that (as with Hyperoptic and Virgin which I've used before) if you pay extra for a static IP they will usually then assign a proper public IPv4 to you, so I will check for that.

You should double check, but I'm led to believe that CF doesn't provide static IPs on their resi plans and only offers a static public IP on business plans.

The feedback on the >= 500 Mbps resi tiers is that the DHCP assigned public IP is fairly sticky.

Or as said go IPv6

Annoyingly with CF on a business service if you ask for a static IP you lose IPv6.

Ah yes recall now that wrinkle. That's pretty batty!

In reply to a post by karypid:

If the ONT is separate, does it provide a public IP address that is presented via DHCP on its Ethernet side (where you'd plug the WAN port of your router)?

Depends on the network operator you sign up with, some use PPPoE, some are DHCP. If you are on a neutral operator (Openreach / CityFibre) then it may depend on the ISP you choose as well as the operator. The likes of CommunityFibre are the network operator and ISP in one (as with Virgin Media or HyperOptic).

In reply to a post by karypid:

Ok I understand now, but would a simple ONT provide an Ethernet port that "bridges" the public IP network to the house (the same way my Virgin equipment provides a public Internet IP when running in modem mode)

Yes: the ONT is a layer 2 device which bridges your ethernet frames to the PON network. Your PPPoE or IP/DHCP frames are carried transparently, just like through a switched network. In the case of a wholesale network like Cityfibre, these frames are bridged all the way through to the final ISP.

IP addressing is therefore an issue only for the WAN port of your router, and the BRAS terminating your session at the ISP side.

In reply to a post by karypid:

or are you fully restricted to a NATed service?

No: the ONT knows nothing about IP, and does not do any routing or NAT.